1. Field of the Invention
The present invention relates to a power lens having an automatic focusing device or a power zoom system, etc., and a camera system having such a power lens.
This application is related to the commonly assigned applications U.S. Ser. No. 07/410,880, Ser. No. 07/652,038, Ser. No. 07/407,424, Ser. No. 07/414,477, Ser. No. 07/414,478, Ser. No. 07/670,733, Ser. No. 07/406,523, and Ser. No. 07/414,476, the disclosures of which are expressly incorporated by reference herein.
2. Description of Related Art
There are various known lens shutter type cameras having automatic focusing devices and power zoom lenses in which zooming is effected by a motor. In an automatic focusing device in a single lens reflex camera, a focusing lens unit is driven by a motor in accordance with a detection result of a focus detecting device incorporated therein. In a power zoom lens, a zooming lens unit is driven by a zoom motor. It is known to drive the motor at high speed, to move the focusing lens unit to an intended position as quickly as possible, when a large displacement of the focusing lens unit is necessary. To prevent the focusing lens unit from moving beyond the intended position, the motor speed is reduced as the displacement of the focusing lens unit decreases. Namely, stepped control of the rotational speed of the motor in accordance with the displacement is carried out.
In a conventional speed control of the motor, as mentioned above, the control speed is determined by the duration of pulses generated in association with the rotation of the motor. I f there is no pulse outputted within a predetermined set time, electrical power is supplied to the motor. If the pulses are outputted within the predetermined set time, power supply to the motor is stopped.
However, in the known control method, there is a frequent change in power supply time and non-power supply time, resulting in an unsteady fluctuation of the motor speed and unsteady operation of the lens unit. Namely, even if the average speed of the motor is constant over a period of time that is substantially longer than a single pulse duration, a large change in the motor speed occurs over a short period of time (corresponding to one pulse or a couple of pulses), thus resulting in an uneven operation of the lens unit.